1) The play/pause key will always play/pause iTunes if it is open, irrespective of whether it is the active window.
2) The play/pause key will ALSO play/pause Decibel if it is the active window.
3) If one of these players is currently playing a song while the other is paused, it will function as described in 1 & 2 above to enact the play/pause instruction, i.e. if Decibel is the active window, it will play the player that is paused, and pause the player that is currently playing a song. Which makes for an interesting effect!

The adjacent advance and back-up keys for navigating through a playlist function the same as 1 & 2 above.

Yes, this is mostly intentional. Apple basically steals the play/pause keys for iTunes, and while other apps can tap in to the key events, there is not an easy way to remove iTunes from the loop. I decided to have the play/pause keys only affect Decibel when it is the frontmost application because it seemed the best compromise.